The present invention relates to battery charger circuits and, more specifically, to a battery voltage equalizer for equalizing the voltage across each of a plurality of series connected charging batteries.
In order to obtain optimum life from lead-acid batteries, the batteries must be correctly charged. When a single charger is used to charge a string of batteries in series, it is unlikely that all of the batteries will receive proper charging, even if all of the batteries are brand new. As a result, some batteries may receive insufficient charge while others receive excess charge. Both of these conditions cause the premature failure of the batteries.
Typically, in series battery string applications, the charger monitors total string voltage rather than individual battery voltages. Because the total string voltage is the sum of the ideal individual battery charging voltages, one battery may receive insufficient charge while another is overcharged. Both overcharging and undercharging, caused by high and low float voltages respectively, damage the batteries and decrease the battery""s life. Overcharging produces excessive heat that can cause the battery plates within the cells to buckle and shed their active material. Undercharging causes buildup of unwanted chemicals on the battery plates, reducing the battery""s capacity and effective life. Unlike NiCad batteries, lead-acid batteries require constant charging with a float voltage level specified by the manufacturer. To prevent damage to the batteries, battery manufacturers typically recommend a charging voltage between 13.25 to 13.65 Volts at 25xc2x0 C. during initial charging. However, except in the case of single-battery applications, this recommendation is rarely observed. To complicate matters, the requirements for a given battery also vary with temperature. For example, MK batteries recommend xe2x88x9216.2 mV adjustment to the float voltage for one xc2x0 C. temperature change.
In a conventional battery charging circuit, a battery charger may be connected in series with a plurality of batteries. For example, as shown in FIG. 1, utilizing three batteries, battery A, battery B, and battery C, 110, 120, 130 connected in series with a battery charger 105. In this example, a 41.1 V battery charger 105 is intended to provide a float voltage of 13.7 V on each battery A, B, and C. When the charging cycle starts, a charge current is supplied to all of the discharged batteries in series. In constant voltage charging, the total string voltage is monitored to determine if all of the batteries have reached the required float voltage. In this example, the required float voltage for each battery is 13.7 V. The charging circuit will operate in float mode when total battery string voltage is 41.1 V (3*13.7V). However, if the batteries have uneven float voltages, as is nearly always the case, then the batteries will not receive the proper charge. For example, battery A may have a float voltage of 13.9 V while battery B has a float voltage of 13.5 V, and battery C has a float voltage of 13.7 V. The total string voltage is still 41.1V, but only battery C is being properly charged. In this example, battery A is being overcharged and battery B is not receiving adequate charge. Overcharging produces excessive heat which can damage the battery. Undercharging causes unwanted chemical buildup. Both of these problems reduce the life of the battery.
The float voltage is the voltage across a battery 110, 120, 130 while the battery 110, 120, 130 is charging in float mode or trickle mode. A typical battery charger 105 switches from normal charging mode to float mode or trickle mode once the charging battery or batteries 110, 120, 130 reach a full charge. In many systems, the typical operating mode is float mode or trickle mode. This is especially true for systems in which the batteries are used for backup power. In such systems, the batteries are fully charged except at initial start-up and following an interruption in the primary power source. In systems where the batteries are used to provide backup power, it is important that the batteries remain in a state of full charge so that the batteries are ready to provide power to the system until the main power supply is restored.
Typically, during the initial charging of a series of batteries to float mode as discussed above, a relatively large constant charging current is applied to the battery string for a relatively short period of time. For example, a charger may supply 10 amps of current for three hours to charge the battery string and reach float mode. Once the charger reaches float mode i.e., the desired string voltage is achieved, such as the 41.1 V string voltage in the example above, a large charging current is no longer needed. However, a small charging current, such as 0.6 amps, is typically applied in float mode to keep the batteries fresh and charged. Unlike the short time period of initial charging, however, the batteries may be charged in float mode for extended time periods such as several months or even years. During this extended time period the individual voltages of the batteries tend to vary, with some battery voltages moving higher than the desired float voltage and some voltages moving lower than the desired float voltage. This may occur even if the individual batteries are initially charged to their desired voltage. These voltage variances, even if not large, may damage the batteries over the extended time period.
Therefore, there is a need in the art for a system and method for charging each of a plurality of batteries in a battery string to a desired voltage which is simple and energy efficient and which does not require the monitoring of individual battery voltages.
The present invention overcomes the above-described problems in the prior art by providing a system and method for charging a plurality of batteries in series to a desired voltage. The present invention also provides a system and method for equalizing the voltage of series connected batteries.
The method and system of the present invention is for use with a plurality of series connected batteries. In the present invention, an equal string current is drawn from each battery of the battery string and redistributed to the batteries by a plurality of secondary charging currents based upon each battery""s comparative voltage. Thus, the size of the secondary current provided to a particular battery varies inversely to the battery""s comparative voltage, such that batteries with a lower voltage are provided with a larger secondary current and batteries with a higher voltage batteries are provided with a smaller secondary current. In this way, the voltages of the batteries are equalized. The total string voltage may be regulated by a charger to ensure that the total string voltage is maintained at a target voltage that is equal to the sum of the desired voltage of the plurality of batteries. Thus, as the total string voltage is maintained at the target voltage, the string current is redistributed unequally to the batteries such that the battery voltages become equalized at the same desired voltage. Other objects, features, and advantages of the present invention will become apparent upon reading the following detailed description of the embodiments of the invention, when taken in conjunction with the accompanying drawings and appended claims. dr
FIG. 1 is a block diagram of a prior art charger.
FIG. 2 is a block diagram of an exemplary embodiment of the present invention.
FIG. 3 is a schematic diagram of an exemplary embodiment of a present invention.
FIG. 4 is a schematic diagram of exemplary embodiment of the present invention in an exemplary operating environment.
FIG. 5 is a plot of the voltage values of four batteries in float mode over time without the system of the present invention.
FIG. 6 is a plot of the voltage values of four batteries in float mode over time using the system of the present invention.